The present invention relates to a melt-transesterification or redistribution process for forming a polycarbonate having chemically combined UV screening units. More particularly, a bisphenol and a diarylcarbonate is melt polymerized with a polyfunctional UV screening monomer such as 1,1-bis(4-hydroxyphenyl)-1-[4-hydroxy-3-(N-2-benzotriazole)phenyl]ethane to provide a polycarbonate having enhanced weathering resistance.
Benzotriazoles have long been recognized for their value as UV screeners when used as part of a silicone top coat as shown for example by U.S. Pat. Nos. 4,061,652 and 4,316,033.
Attempts to directly introduce a benzotriazole compound, such as an aromatic polyhydric phenol having at least one chemically combined benzotriazole group, as a UV screener into a polycarbonate by pelletizing a blend of the benzotriazole compound with the polycarbonate has had limited success. Extrusion can result in a product having the UV screener uniformly dispersed throughout the final resin. The net result is that the UV screener merely functions as a diluent, since most photo-degration of neat polycarbonate occurs within the first 75-100 .mu.m of the surface. In addition, since the UV screener is merely dispersed in and not chemically bound to the polymer backbone, loss due to juicing, plate-out, miscibility/compatibilization, and volatilization are often major problems or concerns. High weight % incorporation of aromatic polyhydric phenol having at least one chemically combined benzotriazole group via an interfacial synthesis has proven to be impossible, since gellation of the polycarbonate occurs as soon as the concentration of the UV screener exceeds 1 mole percent. A more efficient method for introducing aromatic polyhydric phenol having at least one chemically combined benzotriazole group into polycarbonate is therefor needed.